The objective of the proposed research will continue to be to expand knowledge and understanding of the interaction of ecological and environmental factors that allow arboviruses to be maintained in their basic transmission cycles and to be transmitted tangentially to human populations. Research will provide a basis for develoment of alternative or improved approaches to control arboviral diseases and for arboviral surveillance. Special attention will be given to the influence that the "greenhouse effect" may have on arbovirus transmission cycles. To achieve this goal, six short-term and interrelated Specific Aims are proposed for intensive study in the next five years: 1) To assess the possible future impact of the "greenhouse effect" on arboviral ecology in southern California and the central valley of California by studies on the effect of temperature on selected biological traits of Culex tarsalis and its vector competence for western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses; 2) To increase our knowledge of the relation of Cx. tarsalis survival, gonotrophic cycles, autogeny, diapause, dispersal and vector competence to summertime activity and overwintering of WEE and SLE viruses by field studies at selected sites in the Coachella and Imperial valleys of southeastern California; 3) To evaluate the hypothesis that acute or relapsing avian malarial infections in immunosuppressed birds will reactivate chronic or latent WEE and SLE viral infections in such hosts; 4) To conduct studies on the effect of environmental conditions during preimaginal development on the expression of vector competence in adult female Cx. tarsalis for WEE and SLE viruses; 5) To elucidate mechanisms which control susceptibility and resistance of Cx. tarsalis to WEE virus, including presence and absence of receptor sites on mesenteronal microvillar membranes and modulation of WEE viral titers; and 6) To develop dynamic system models to aid in the understanding of the underlying mechanisms involved in the maintenance and transmission of WEE virus.